1. Field of the Invention
The invention generally relates to data communications receivers and operation methods, and in particular to the correction of a frequency and/or phase error of an incoming digitally modulated signal.
2. Description of the Related Art
A WLAN (Wireless Local Area Network) system is a flexible data communications system implemented as an extension to or as an alternative for, a wired LAN. Using radio frequency or infrared technology, WLAN systems transmit and receive data over the air, minimizing the need for wired connections. Thus, WLAN systems combine data connectivity with user mobility.
Today, most WLAN systems use spread spectrum technology, a wide-band radio frequency technique developed for use in reliable and secure communication systems. The spread spectrum technology is designed to trade-off bandwidth efficiency for reliability, integrity and security. Two types of spread spectrum radio systems are frequently used: frequency hopping and direct sequence systems.
The standard defining and governing wireless local area networks that operate in the 2.4 GHz spectrum, is the IEEE 802.11 standard. To allow higher data rate transmissions, the standard was extended to 802.11b that allows data rates of 5.5 and 11 Mbps in the 2.4 GHz spectrum. This extension is backwards compatible.
When operating a WLAN receiver, code synchronization is necessary because the code is the key to despreading the desired information. A good synchronization is achieved when the coded signal arriving at the receiver is accurately timed in both its code pattern position and its rate of chip generation.
Generally, the synchronization process performed in any receiver can be divided into two phases. First, a synchronization acquisition is performed in to initially synchronize the receiver with a received signal. The second part of the synchronization follows the initial acquisition since the receiver must continue to operate in such a way that it remains locked with its code reference. That is, the receiver exactly tracks the coded incoming signal to cause its own code chip rate to match the incoming code chip rate as precisely as possible.
With respect to the synchronization algorithms used, receivers may be classified into data-aided and non data-aided receivers. The data-aided approach does not require a prior knowledge of the interference parameters but requires a training data sequence. Non data-aided (or blind) algorithms require no training data sequence but only knowledge of the desired user signal sequence and its timing.
In WLAN systems as well as in other spread spectrum communication systems, the signal on its way from the transmitter to the receiver experiences several distortions. A frequency or phase error may result from a frequency or phase offset of the radio frequency oscillators at the transmitter and the receiver. It may be the task of any synchronization unit within the receiver to perform an error correction, no matter if in the acquisition phase or in the tracking phase.
Turning now to FIG. 1, an error correction arrangement is schematically shown that comprises a frequency error correction unit 100 and a phase error correction unit 110. The frequency error correction unit 100 is used to compensate for the frequency difference, and the phase error correction unit 110 will then compensate for the residual phase error. Thus, the phase error correction unit 110 has the task to remove the remaining phase error such that the received signal is as close as possible to the transmitted signal, to minimize the probability of demodulation errors.
Error corrections circuits in existing data communications receivers such as WLAN receivers still have a number of problems. One problem is that conventional circuits often are highly involved and therefore lead to high circuit development and manufacturing costs. Moreover, such circuits usually require non-linear operations to be performed which are difficult to implement. Another disadvantage of existing circuits may be that the conventional adjustment processes may sometimes not be performed with sufficient phase or frequency resolution, and are restricted in use by the individual capabilities of the respective hardware implementation.